We will continue studies on the mechanism of action of maytansinoid compounds on cells and on tubulin. For this we have synthesized the radioactive derivative maytanacine which is almost as potent as maytansine in our test systems. We will investigate the binding of this compound to tubulin derived from sea urchin eggs, brain, CHO cells and KB cells. The latter two cell lines differ in sensitivity to the drug by about 15 times. This difference could be due to the sensitivity of the tubulin to the drug which we wish to test. It could also be due to differential uptake or differential destruction of the drug. We will test the uptake properties of CHO versus KB cells and also see whether the compounds are metabolized differently in these cases. We will also continue studies on the isolation of temperature sensitive maytansine resistant mutants in studies on the uptake mechanisms for this and related drugs. Finally, we will continue to explore methods for the preparation of very highly radioactive maytansine derivatives which will be required in studies of the binding sites of these compounds in cells.